The present invention relates to a method of making rigid, stiff, reinforced thermoplastic pipe. Thermoplastic pipe made of solid plastics such as polyvinyl chloride (PVC) and graft acrylonitrile-butadiene-styrene (ABS) is sold in huge quantities for use in water supply, sanitary and storm pipes, as well as many other applications. One great advantage of the thermoplastic pipe is that it is lighter in weight than the pipe it usually replaces or displaces; another advantage is its ease of fabrication into fluid systems. Its structural integrity is good for many applications, but improvement is desired in other applications.
For many applications a desirable pipe is a smooth surfaced, fluid impervious pipe of a thickness to give it good structural integrity, but light enough to have the light-weight and ease of fabrication advantages of plastic pipe. A simple, inexpensive and novel method of making such a pipe and the pipe made by such method is the subject matter of this invention.
The pipe made in accordance with the method of the present invention is a lightweight, rigid pipe having spaced thermoplastic exterior and interior tubes and a buckled or corrugated skin or ribbon occupying the space between the exterior and interior tubes. The corrugated skin acts to space, brace and reinforce the pipe made from the two tubes. The exterior and interior tubes, as well as the buckled skin, are preferably of the same thermoplastic material. The tubes and the ribbon may, however, be of different thermoplastic resins.
The manufacture of the present pipe requires the use of two extruders feeding to a common pipe tool or die. Appropriate passageways are provided in the die for receiving the plastic from one extruder and forming the interior and exterior tubes and for receiving plastic from another extruder to form the buckled skin. The interior and exterior tubes are spaced, for example, 0.125 of an inch for a 4" OD pipe, and are held apart by pulling a vacuum on the tubes as they exit the extruder die. The ribbon or skin is extruded into this space at a linear rate exceeding the rate of the plastic which forms the interior and exterior tubes. The ribbon will be unsupported, and it either swells and touches the interior wall of the exterior tube or collapses and touches the exterior wall of the interior tube. In either case, the ribbon will touch the tube surfaces, buckle and provide the corrugations.
The apparatus employed for carrying out the method of the present invention comprises a plurality of extruders, preferably two, connected to a single die having multiple passages for receiving and forming the plastic discharged by the extruders.